1. Field of the Invention
The present invention relates to an image forming apparatus equipped with an intermediate transfer body which is wound around a photosensitive body and rotated by being driven by the rotation of the photosensitive body. In the image forming apparatus, the intermediate transfer body is rotated more than once, respective color toner images are superimposed on the intermediate transfer body with each complete circuit of the intermediate transfer body, and thereby a full-color toner image is formed on the intermediate transfer body.
2. Description of the Related Art
In a four cycle type full-color laser printer in which a full-color toner image including yellow (Y), magenta (M), cyan (C), and black (K) colors are formed on an intermediate transfer belt with one photosensitive body, the intermediate transfer belt makes four turns (i.e., is rotated four times), and respective color toner images are superimposed on the intermediate transfer belt with each turn of the intermediate transfer belt. Therefore, in the four cycle type full-color laser printer, in order to suppress generation of color shift, a fluctuation (variation) in rotation period of the intermediate transfer belt is detected, and the timing of the start of sub-scanning to a photosensitive body (drum) by an exposure device is controlled according to the fluctuation.
A fluctuation in load applied to the intermediate transfer belt due to a transfer roller, a cleaning blade or a cleaning roller contacting with or separating from the intermediate transfer belt, can be a factor in the fluctuation in the rotation period of the intermediate transfer belt. Conventionally the start timing of the sub-scanning to the photosensitive drum by the exposure device is controlled in consideration of the fluctuation in the load applied to the intermediate transfer belt (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2004-302308).
A change in drive radius of the photosensitive drum due to a change in the amount of toner which is electrostatically adsorbed onto the photosensitive drum, can be another factor in fluctuation in the rotation period of the intermediate transfer belt. The change in drive radius of the photosensitive drum occurs in a configuration in which the intermediate transfer belt is wound around the photosensitive drum and rotated by being driven by the rotation of the photosensitive drum. As shown in FIGS. 7A and 7D, when a small amount of toner is electrostatically adsorbed to a photosensitive drum 50, a drive radius R0 of the photosensitive drum 50 is substantially equal to an actual radius R of the photosensitive drum 50, and a moving speed Vb of an intermediate transfer belt 64 is substantially equal to a rotation speed Vd of the photosensitive drum 50. On the contrary, as shown in FIGS. 7B and 7C, when a large amount of toner is electrostatically adsorbed to the photosensitive drum 50, the drive radius R0 of the photosensitive drum 50 is increased by a thickness of toner, and the moving speed Vb of the intermediate transfer belt 64 becomes faster than the rotation speed Vd of the photosensitive drum 50. This is clear from the fact that, when performing a full-color print with a Y color coverage rate ranging from 2 to 3%, a M color coverage rate of 100%, a C color coverage rate of 100%, and a K color coverage rate ranging from 2 to 3%, the rotation period of the intermediate transfer belt 64 when performing primary transfer of the M color and C color becomes shorter than the rotation period of the intermediate transfer belt 64 when performing primary transfer of the Y color and K color.
Therefore, as shown in FIGS. 8A and 8B, when a small amount of toner is electrostatically adsorbed to the photosensitive drum 50, for example, in a case in which a pitch of a toner image T on the photosensitive drum 50 is L0, the pitch on the intermediate transfer belt 64 also becomes L0. On the other hand, as shown in FIGS. 8C and 8D, when a large amount of toner is electrostatically adsorbed to the photosensitive drum 50, even if the pitch of the toner image T on the photosensitive drum 50 is L0 similarly to the former case, the pitch on the intermediate transfer belt 64 becomes L1 (>L0). Accordingly, even if the start timing of sub-scanning is adjusted for each color, the color displacement will gradually increase while transferring the toner image from the photosensitive drum 50 to the intermediate transfer belt 64.